1. Field of the Invention
The present invention relates generally to a method and apparatus for detecting registering errors of register marks printed on a paper web by each printing section to maintain exact agreement in the printing position of each color, and an automatic register control apparatus operating to eliminate detected registering errors, and more specifically to a method and apparatus for detecting registering errors, and an automatic register control apparatus that make it possible to detect the position of register marks more accurately when the shape of printed register marks is deformed.
2. Description of the Related Art
In a multi-color rotary press, if the positions of images of each color printed by each printing section do not agree with each other, no multi-color printed matter having desired color tones can be obtained. Efforts are therefore made in the trade to detect register errors that correspond to color mismatching, and to correct them to reduce registering errors to zero.
The most commonly used registering error detecting methods is such that register marks are printed, together with print images, for each color on a paper web, a register mark is set as a reference, and a deviation of the relative position of the other register marks with respect to the position of the reference register mark is detected as an amount of registering error. A reference line or point is therefore set on each register mark to detect deviations in the relative position.
Among publicly-known methods for setting a reference line or point in each register mark proposed is Japanese Published Unexamined Patent Application No. Sho-58(1983)-20457, in which a register mark comprising a line segment having a predetermined length is provided for each color on a paper web in parallel with the longitudinal direction (direction of web travel) and at predetermined intervals in the lateral direction (across-the-width direction of web), a longitudinal reference line is set at the tip in the longitudinal direction of each register mark and a lateral reference line is set at the center of the line width of the register mark, and a deviation in relative position between the reference register mark and the other reference marks is found.
Proposed in Japanese Published Unexamined Patent Application No. Hei-1(1989)-192558, is a method for finding a deviation in relative position between a reference register mark and the other register marks by reading cross-shaped register marks printed on a paper web by a color camera, scanning color-separated still images from top to bottom in the longitudinal direction and in the lateral direction, setting the first acquired image position (upper left edge of the vertical line of the cross-shaped register mark) as the lateral reference, and the lateral image position (upper edge of the vertical line of the cross-shaped register mark) acquired as the side edge of the first acquired image (left edge of the vertical line of the register mark) is scanned from top to bottom in the longitudinal direction as the longitudinal reference, and setting the intersection of a line parallel to the longitudinal direction passing the lateral reference and a line parallel to the lateral direction passing the longitudinal reference as the reference point of the register mark.
Furthermore, proposed in Japanese Published Unexamined Patent Application No. Hei-3(1991)-11900 is a method for finding a deviation in relative position between a reference mark and the other reference marks by using register marks of a right-angled triangle shape having sides parallel to the longitudinal and lateral directions, detecting the register marks in the longitudinal direction with a photoelectric sensor, providing a predetermined number of signal pulses output at every rotation of a plate cylinder and a reference pulse output at every rotation of the plate cylinder, setting the side of the register mark parallel to the lateral direction of the register mark as the longitudinal reference line and the hypotenuse of the register mark as the lateral reference line, detecting the number of signal pulses output in a lapse of time from the generation of the reference pulse till the photoelectric sensor detects the longitudinal and lateral reference lines, and finding a deviation of the number of the detected signal pulses from a predetermined number of reference pulses and converting the deviation into a distance.
All of these prior-art methods involve the detection of the outer edge of an image of a printed register mark as a reference line or point in both the longitudinal and lateral directions or either thereof.
Images of printed register marks, however, tend to involve blurs on the outer edges due to the adverse effects of secular changes in ink viscosity, change in the surface quality of paper, and secular changes in the balance between ink and dampening water particularly in offset printing, and the amount of blurs tends to change over time and by location. The longitudinal reference line in Reference 1, the reference point in Reference 2, and the longitudinal and lateral reference lines in Reference 3 have therefore had the difficulty in detecting correct deviations due to the negative effects of blurs.
To cope with this, methods for setting reference points free from the negative effects of blurs have been proposed and publicly known, for example, in Japanese Published Unexamined Patent Application Nos. Sho-63(1988)-2265 1, Hei- 1(1989)-192559, Hei-7(1995)-246700 and Hei-7(1995)-304162.
Proposed in Japanese Published Unexamined Patent Application No. Sho-63(1988)-22651 is a method for eliminating the negative effects of blurs by scanning register marks of a 45-degree inclined square shape in the lateral direction with a line sensor, finding the central coordinate values of a plurality of the acquired line segments crossing the register marks, regarding the average value of the central coordinate values as the lateral central coordinate value of the register mark, applying to it a geometrical figure rule that, in a 45-degree inclined square, the distance from a point on the straight line joining the vertices to the hypotenuse is equal to the distance up to the vertex closer to it to find the closer longitudinal vertex position based on the lateral central position, determining two longitudinal vertices by carrying out the above procedure at multiple location, regarding the midpoint of the vertices as the center of the register mark, and setting the midpoint as a reference point.
Also proposed in Japanese Published Unexamined Patent Application No. Hei-1(1989)-192559 is a method for eliminating the negative effects of blurs by using cross-shaped register marks, parallel-scanning the pixel matrix of the still images of the register marks in longitudinal and lateral directions, adding up the number of pixels appearing at each scanning position in both directions, regarding the intersection of a scanning line having the largest number of pixels as the center of the cross-shaped register mark, and setting it as a reference point.
In addition, Japanese Published Unexamined Patent Application No. Hei-7(1995)-246700 proposes a method for eliminating the negative effects of blurs by scanning circular register marks at predetermined intervals in the longitudinal direction and in the lateral direction at two locations on both sides of the diameter with a line sensor, regarding the central coordinate value of the line data of the scanning line crossing the register mark as the lateral center coordinate value of the circular register mark, calculating the longitudinal coordinate value of the circle center from the lengths of the two scanning line data using the Pythagorean theorem, finding the central point coordinate value of the circle from the longitudinal coordinate value of the circle center and the lateral center coordinate value found earlier, and regarding it as a reference point.
Furthermore, Japanese Published Unexamined Patent Application No. Hei-7(1995)-304162 proposes a method for eliminating the negative effects of blurs by scanning register marks of a right-angled isosceles triangle shape with the hypotenuse oriented in the longitudinal direction at two longitudinal locations at predetermined intervals so as to cross the different sides containing the right angle in lateral direction with a photoelectric sensor, calculating the hypotenuse length and the right-angle vertex position of the right-angled isosceles triangle by applying a geometrical figure rule that, in a right-angled isosceles triangle with the hypotenuse oriented in the longitudinal direction, the length of the lateral scanning line from a point of the hypotenuse is equal to the distance from the point to a longitudinally closer vertex, finding the calculation results the center of gravity coordinate value of the right-angled isosceles triangle, and regarding it as a reference point.
These proposals for eliminating the negative effects of blurs are intended to improve the accuracy of detection of registering errors by calculating a center or center of gravity of a register mark using geometrical figure rules based on the positional information obtained from the outer edge of the printed register mark that contains blurs, and setting the center or center of gravity as a reference point. The calculation method using geometrical figure rules, however, cannot calculate a correct reference point when a deformation of the register mark changes similarly with respect to the reference point, that is, unless blurs remain uniform over the entire outer edge of the register mark.
In actual printing operation, however, an image of a printed register mark tends to be subtly changed due to secular changes in ink viscosity, changes in the surface quality of paper, and the effects of dampening water in offset printing. In offset printing, a deteriorated balance between ink and dampening water may cause contamination on non-image areas, slurs of images, local blurs, missing images and blurs. Thus, deformation of a register mark tends to become uneven over the entire surface. In such a case, automatic register control may result in aggravated registering errors, increasing waste printing.
There has been a strong need among printing machine users for achieving close registration as quickly as possible to minimize waste printing at the start of printing. Because of the difficulty in accomplishing good quality printed matter until the supply of ink and dampening water is stabilized at the start of printing, however, it has been often the case that automatic register control has been left inoperative for a given time after the start of printing. As a result, even when all other printing conditions have got ready for operation, printing registration has not been able to be accomplished for a predetermined time after the start of printing due to suspended automatic register control. This has resulted in the discarding of waste printed matter that had otherwise been shipped as normal printed matter, contrary to printing machine users.
The present invention seeks to overcome these problems by improving the accuracy with which the reference position of deformed register marks is detected, thereby reducing waste printing in unstable printing conditions, particularly at the start of printing.
It is an object of the present invention to provide a method for detecting registering errors in multi-color rotary presses that can improve the accuracy with which the reference position of deformed printed register marks is detected.
It is another object of the present invention to provide a method for finding a first center of gravity that is an approximate center of gravity.
It is still another object of the present invention to provided a method for finding a first center of gravity that is an approximate center of gravity from the read matrix data of register marks of a single and point-symmetrical figure.
It is a further object of the present invention to provide a method for finding a first center of gravity that is an approximate center of gravity from the read matrix data of register marks of a single and point-symmetrical figure, and finding a second center of gravity that is a high-precision center of gravity from the first center of gravity.
It is a still further object of the present invention to provide an apparatus for detecting registering errors in multi-color rotary presses that can improve the accuracy with which the reference position of deformed printed register marks is detected.
It is a still further object of the present invention to provide an automatic register control apparatus in multi-color rotary presses that can improve the accuracy with which the reference position of deformed printed register marks is detected.
In the embodiments disclosed in the present invention, a method for detecting registering errors in multi-color rotary presses comprising the steps of printing more than one register mark by each printing section on a paper web, causing a light source to flash based on a reference signal output by signal output means that operates in synchronism with the rotation of a predetermined reference plate cylinder and outputs signals, reading the register marks printed by all the printing sections by reading means that operates in synchronism with the flashing of the light source, developing the read register marks into matrix data, finding a first center of gravity that is an approximate center of gravity of each register mark, further finding a second center of gravity that is a high-precision center of gravity of each register mark from the matrix data based on the first center of gravity, using the second center of gravity of a predetermined register mark and finding the relative positions of the second center of gravity of the other register marks, finding a deviation of the found relative positions from the predetermined reference relative position and regarding the deviation as a register error.